In the process of extruding cellular plastic foams, solid plastic particles, typically small solid cubes of the desired plastic, and if desired various compounding ingredients included therein, are fed into a primary extruder where they are propelled along the extruder barrel by action of the extruder feed screw. The energy imparted by the action of the feed screw melts the plastic into a homogeneous mass, whereupon a blowing agent is introduced through an inlet port in the side of the barrel. After incorporation of the blowing agent, the composition is cooled in a secondary extruder to the point at which its viscosity is increased sufficiently to extrude the melt through a forming die into the desired finished shape.
In the past, cooling has frequently been accomplished by feeding the foamed melt into an additional extruder, often termed a "secondary" foam extruder, wherein the melt is propelled along an extruder barrel by means of a conventional feed screw, being cooled in the process by heat transfer through the wall of the barrel. Unfortunately, however, some of the melted mass located in the space provided for necessary clearance between the barrel wall and the feed screw remains essentially stationary, providing an insulating barrier which decreases heat transfer through the barrel wall. Furthermore, the stationary mass is ordinarily present in the form of a semisolid, or solid, which acts as a brake on the feed screw, resulting in more energy being required to turn it, and in the additional generation of unwanted heat, due to frictional effects.
In the prior art, attempts have been made to improve the heat transfer of heat exchanger devices through provision of scraping implements. One approach is such as shown in U.S. Pat. No. 4,185,352.
In view of the foregoing, therefore, it is a first aspect of this invention to provide a heat exchange device, herein termed a "cooler extruder," which provides greatly improved heat transfer, thereby allowing higher production rates to be achieved.
A second aspect of the invention is to provide a cooler-extruder which requires less energy consumption than alternative heat transfer devices.
An additional aspect of this invention is to make available a cooler-extruder whose wiper section employes wiper blades spaced apart from the shaft which rotates them, thereby providing an opening which allows the blades to be rotated through the plastic melt with less overall expenditure of energy.
Another aspect of this invention is to provide a cooler-extruder with easily replaceable parts at points of high wear.
A further aspect of the invention is the provision of a cooler-extruder with more economical, smaller components, including its drive motor, cooling jacket, barrel, and feed screw.
An added aspect of the invention is to permit higher rates of extrusion to be achieved by providing more rapid cooling of a plastic melt in a jacketed extruder barrel.